1. Field of the Invention
This invention relates to a multiple feed detection device for detecting a feed of two or more overlapped sheets (multiple feeds) when sheets are carried along a carriage route.
2. Description of the Related Art
A sheet feed mechanism provided in, e.g., a printing machine, separates sheets loaded on a sheet feed table one by one and carries them, but two or more sheets may be carried in an overlapped state when the sheets are fed to printing drums.
Therefore, the multiple feed detection at the time of carrying the sheets is conventionally conducted by arranging a multiple feed sensor consisting of an optical sensor of transmission type over a carriage path along which the sheets are carried, and detecting the light transmission quantity depending on the paper thickness of the sheets by the multiple feed sensor. The accuracy of the multiple feed detection can be improved in such a method by covering a wide measurement range during the sheet feed detected by the multiple feed sensor (an optical sensor of a transmission type) to increase the sampling number.
Most of the sheet feed mechanisms equipped in, e.g., printing machines, carry the sheets by using two or more types of rollers which differently operate. FIGS. 5A and 5B show a view of an example of a sheet feed mechanism having a multiple feed sensor.
A sheet feed mechanism 1 shown in FIGS. 5A and 5B includes a first carrier member 5 consisting of a scraper roller 3 and a pickup roller 4 which operate for picking up each sheet 2, and a second carrier member 8 consisting of a guide roller 6 and a timing roller 7 which operate for obtaining the right timing of the sheet feed.
A multiple feed sensor (a sheet detector) 10 for detecting the multiple feed of the carried sheets 2 is provided around a carriage path 9 between the first carrier member 5 and the second carrier member 8.
The multiple feed sensors 10 are constituted by optical sensors of transmission-type including a light emitting sensor 10a and a light receiving sensor 10b. The light emitting sensor 10a is, for example, constituted by a light emitting diode, a laser diode, or a lamp. The light emitting sensor 10a is disposed at a predetermined distance apart from the carriage path 9 along which the sheets 2 are fed.
The light receiving sensor 10b is, for example, constituted by a photodiode. The light receiving sensor 10b is disposed to be opposed to the light emitting sensor 10a at a predetermined distance apart from the carriage path 9, e.g., in an equally spaced apart relation between the light emitting sensor 10a and the carriage path 9 such that the carriage path 9 along which the sheets 2 are carried is sandwiched between the sensors.
If the sheets 2 are not carried, the light emitted from the light emitting sensor 10a is directly received by the light receiving sensor 10b in the multiple feed sensor 10, whereas if the sheets 2 are carried, the light transmitted through each of the sheets 2 is received by the light receiving sensor 10b. 
In the sheet feed mechanism 1, the sheets 2 loaded on a sheet feed table (not shown) are picked up one by one from the top by the first carrier member 5 so as to be carried, and the sheets 2 picked up by the first carrier member 5 are carried toward a printing drum (not shown) by the second carrier member 8. Whether or not the sheets 2 carried from the first carrier member 5 to the second carrier member 8 are carried in an overlapped state is then determined based on detected signals of the multiple feed sensor 10.
In a method for measuring the light transmission quantity when the sheets 2 mentioned above pass between the multiple feed sensors 10, generally, the light transmission quantity tends to increase as the distance between the sheet passing position and the light emitting sensor 10a becomes long. Therefore it is necessary to keep the position of the sheet passing between the multiple feed sensors 10, particularly a part of sheet through which the light transmits.
However, in such a construction that the sheets 2 are carried by the sheet feed mechanism 1 mentioned above, a slack of the sheet 2 may occur when the sheet 2 is carried from the first carrier member 5 to the second carrier member 8 as shown in FIG. 5A., and a spring of the sheet 2 may occur when the end of the sheet 2 separates from the first carrier member 5 (the pickup roller 4) as shown in FIG. 5B.
Therefore, in the conventional method, the position of the sheet passing between the multiple feed sensor 10 can not be fixed due to the above mentioned slack and spring of the sheet. Considering this, if the measured area of the sheet passing between the multiple feed sensor 10 is made broader, data sampled within the measured area may include considerable errors. As a result, only the data required to detect a multiple feed must be chosen from the sampled data.
In addition, in the conventional method, the average value of the sampled data with respect to the first sheet is regarded as a reference value, and a comparison with the reference value obtained by the one measurement is performed to detect the multiple feed. For this reason, even if the multiple feed sensor 10 senses a part of the sheet, the part on which a dust or the like adheres, to detect small quantity of the light transmission, this may be erroneously discriminated as an occurrence of a multiple feed.
Accordingly, the present invention has been accomplished in view of the above mentioned problems, and it is an object of the present invention to provide a method and a device for detecting a multiple feed capable of improving the accuracy of multiple feed detection by decreasing the frequency of the multiple feed misdetection.
To achieve the above object, according to an aspect of the present invention, there is provided a multiple feed detection device comprising: a sheet detector having a light emitting sensor and a light receiving sensor arranged in vicinity of a carriage path to detect quantity of light that has transmitted through a sheet; and a processor which controls the sheet detector to detect a predetermined number of samples of light quantities for each of a plurality of sampling ranges, detects a multiple feed for each sample range based on light quantity data sampled for each sampling range, and decides the multiple feed of sheets based on a plurality of results of the detection of the multiple feed for the plurality of sampling ranges.
In a preferred embodiment of the present invention, the plurality of sampling ranges are arranged in the direction of sheet feed, and the processor conducts the samplings for the plurality of sampling ranges sequentially at predetermined sampling start timings by means of a pair of a light emitting sensor and a light receiving sensor.
In a preferred embodiment of the present invention, a plurality of sheet detectors are arranged in a direction perpendicular to the direction of sheet feed.
In a preferred embodiment of the present invention, the processor sets a total number of the plurality of sampling ranges to be odd in advance, and decides the multiple feed when the number of sampling ranges for which the multiple feed is detected is more than half of the total number.
In a preferred embodiment of the present invention, the processor decides that the multiple feed has occurred when successive results that multiple feeds are detected are obtained starting from a front end sampling range on the sheet.
To achieve the above object, according to another aspect of the present invention, there is provided a multiple feed detection method comprising the steps of: arranging a sheet detector having a light emitting sensor and a light receiving sensor in vicinity of a carriage path to detect quantity of light that has transmitted through a sheet; controlling the sheet detector to detect a predetermined number of samples of light quantities for each of a plurality of sampling ranges; detecting a multiple feed for each sample range based on light quantity data sampled for each sampling range; and deciding the multiple feed of sheets based on a plurality of results of the detection of the multiple feed for the plurality of sampling ranges.
In a preferred embodiment of the present invention, the plurality of sampling ranges are arranged in the direction of sheet feed, and the samplings for the plurality of sampling ranges are sequentially conducted at predetermined sampling start timings by means of a pair of a light emitting sensor and a light receiving sensor.
In a preferred embodiment of the present invention, a total number of the plurality of sampling ranges is set to be odd in advance, and it is decided that the multiple feed has occurred when the number of sampling ranges for which the multiple feed is detected is more than half of the total number.
In a preferred embodiment of the present invention, it is decided that the multiple feed has occurred when successive results that multiple feeds are detected are obtained starting from a front end sampling range on the sheet.